Continuously Variable Transmissions (CVT's) are used in applications such as snowmobiles and ATV's to alleviate the need for the driver to shift through a set of fixed gears as the vehicle accelerates through it's range of speeds. Typically the CVT is connected to the output shaft of the vehicle's engine, providing a continuously variable reduction of the relatively higher engine rotation speed, to the relatively lower rotational speed of the vehicles drive system. The drive system could be comprised of either a direct connection from the CVT to the drive axle, or the CVT is used in conjunction with an additional gearbox and/or transmission. The addition of a gearbox is desirable on ATV's to permit the driver to shift between neutral, high, low, and reverse gears. The output shaft of the CVT is connected to the gearbox which, in turn, has an output connected by suitable linkages to the drive axle(s) of the vehicle. Other drive train components, such as differentials, can also be employed between the CVT and drive axle(s) to provide further gearing reduction of the final drive ratio.
Typically a CVT is comprised of a split sheave primary clutch (drive clutch) connected to the output shaft of a vehicle's engine and a split sheave secondary clutch (driven clutch) connected (often through additional drive train components and suitable linkages) to the vehicle's axle(s). An endless, flexible, V-shaped drive belt is disposed about the clutches. Both clutches have a pair of complementary sheaves with one sheave being movable laterally with respect to the other. The lateral position of the movable sheaves in each of the clutches determines the diameter at which the drive belt resides at any moment in time, thus determining the effective gear ratio of the CVT. The drive clutch's sheaves are normally biased apart, such as with a coil spring, so that when the engine is at idle speeds the drive belt is not effectively engaged with the sheaves. As a result there is no driving force transmitted to the driven clutch. The driven clutch's sheaves are normally biased together (e.g., by a torsion spring working in combination with a helix-type cam, as described below), so that when the engine is at idle speeds the belt resides at the outermost diameter of the driven clutch sheaves.
The spacing of the sheaves on the drive clutch is usually dependent upon centrifugal flyweights. As engine speed increases it produces an increase in the rotational speed of the directly connected drive clutch. The increased speed causes the flyweights to apply a force on the movable sheave to urge it towards the stationary sheave. The two sheaves pinch onto the drive belt, causing it to rotate. This, in turn, causes the driven clutch to begin to rotate. An increase in engine speed thus causes a decrease in lateral distance between the drive clutch sheaves. Any decrease in lateral distance between the drive clutch sheaves results in an increase of the diameter at which the drive belt resides about the drive clutch's rotational axis. The clutch, therefore, can be said to be speed sensitive.
As the sheaves of the drive clutch pinch together and force the belt to climb outwardly, the tension of the belt (not being stretchable) pulls itself inwardly between the sheaves of the driven clutch, resulting in a decrease of the diameter at which the drive belt resides about the driven clutch's rotational axis. This smooth movement of the belt inwardly and outwardly on the drive and driven clutches provides seamless changes in the effective gear ratio of the transmission in infinitely variably increments. CVT's are, thus, sometimes referred to as infinitely variable transmissions.
The spacing of the sheaves on the driven clutch is usually controlled by a different mechanism. Although a coil spring could be used to bias the sheaves of the driven clutch together, typically a more sophisticated torque-sensitive system is used to pinch the belt harder as more torque is conveyed by the drive belt to the driven clutch. A generally cylindrical cam with, for example, three cam surfaces (often called ramps) on one end is secured to the output shaft of the driven clutch. Because the ramps are generally helical in shape, this cam is often referred to as a helix. A set of a corresponding number of cam followers (typically buttons or rollers) is mounted to the movable sheave, and the movable sheave is mounted within the driven clutch so that it is free to move laterally and is also rotatable with respect to the shaft. The buttons or rollers are mounted in positions aligned with the ramps of the helix, and a torsion spring typically is used to apply a force that urges the movable sheave rotationally to keep the buttons or rollers engaged against their respective helix ramps.
As torque is transmitted by the drive belt to the driven clutch sheaves, the belt tends to urge the movable sheave laterally away from the stationary sheave, and also tends to rotate the movable sheave with respect to the shaft. Since the buttons are held against the ramps by the torsion spring, the torque being applied by the belt to the movable sheave tends to cause the buttons to slide up the ramps, which in turn tends to push the movable sheave toward the stationary sheave. Thus, the helix converts the torque of the drive belt into a force that pinches the sheaves together, providing good frictional contact between the sheaves and the drive belt. The more torque applied by the belt to the driven clutch, the harder the sheaves of the driven clutch pinch the belt, therefore preventing the belt from slipping under load, and also causing the transmission to downshift for increased power (i.e., urging the belt outwardly between the sheaves of the driven clutch which causes the belt to move inwardly between the sheaves of the drive clutch). Since the spacing of the sheaves in the driven clutch is dependent upon torque, the driven clutch can be said to be torque sensitive.
The actual position of the belt within the sheaves of the drive and driven clutches is determined by the balance of the forces acting on the movable sheaves in the two clutches. In the drive clutch, these forces consist of the coil spring urging the sheaves apart and the speed-dependent force of the centrifugal flyweights, which urges the sheaves together. In the driven clutch, these forces consist of the torsion spring urging the sheaves together along with the torque-dependent force generated by the rollers on the helix ramps.
As mentioned above, the balance of forces acting on the movable sheaves determines the position of the drive belt between the clutch sheaves. In some situations this balance can be disrupted. For example, when the vehicle is traveling along at a given speed and the rider momentarily lets off of the throttle, the speed sensitive drive clutch continues to pinch to the drive belt (since the vehicle's speed is not immediately affected and the engine rpm does not drop instantaneously). At the same moment the torque sensitive driven clutch reduces its pinching force on the drive belt substantially, since the engine torque output drops rapidly. The drive clutch thus overcomes the driven clutch and causes the CVT to tend to up shift. When the rider reapplies the throttle, torque is restored to the driven clutch, but the transmission takes a moment to downshift to the proper gear ratio to accelerate. This downshifting requires the belt to be forced outwardly on the driven clutch, a movement that can be inhibited by the fact that the movable sheave must rotate with respect to the stationary sheave as the torque from the belt causes the rollers to move along the helix ramp. This rotation of one sheave with respect to the other sheave while both in contact with the drive belt, causes scrub on the sides of the drive belt and does not always happen as quickly as would be desired. Accordingly, the vehicle takes a moment to downshift, making it less responsive than would be desirable.
Due in part to the tendency of the CVT to up shift when the rider lets off of the throttle, the CVT does not provide significant engine braking by backdriving the engine. That is, in some types of vehicle drive trains when the vehicle is traveling at a given speed and the throttle is dropped (e.g., to an idle speed), the rotation of the drive wheels of the vehicle will backdrive the drive train causing the engine to rotate at a speed greater than it otherwise would (based on throttle position). The inherent frictional forces present throughout the drive train, including particularly the compression forces present in the engine cylinders, tend to slow the vehicle down. This condition is commonly referred to as engine braking, and can be a desirable and useful feature. The degree of engine braking provided (in vehicles capable of doing so) is dependent upon the gear ratio of the transmission—in higher gears, less braking is provided, and in lower gears relatively more braking is provided. Thus, the tendency of the CVT to up shift when the rider lets off of the throttle makes the CVT less effective in braking the engine to slow the vehicle down.
Conventional CVT systems also do not provide engine braking when the engine is at idle speed. When the engine is simply idling, the primary drive clutch has its sheaves biased apart by a coil spring so that the sheaves do not effectively engage the drive belt. Usually the length of the drive belt is chosen so that it is somewhat loose in the idle position, preventing the vehicle from “creeping”. A consequence of this looseness of the drive belt, however, is that the driven clutch is not capable of backdriving the drive clutch (and, therefore, the engine) when the belt and clutches are in the idle position. This could occur while the vehicle is in motion, but the engine is at idle speed.
Also due to the tendencies of the conventional CVT system to up shift with increasing engine speed, and downshift with increasing torque load, there is a point at which the system can be overloaded and the drive belt will slip on the sheave faces. The drive belt can only transfer a limited amount of torque via contact between itself and the sheave faces of both the drive and driven clutches based on the frictional forces between them. If an overload is applied to the system, engine speed will be higher than idle and the engine will be outputting high torque to the CVT system. The drive clutch will attempt to push the belt outwardly on it's sheaves, while the driven clutch will pinch the belt very tightly and attempt to force it outwardly on it's sheaves as well. The lateral force applied to the drive belt by the driven clutch sheaves will attempt to hold the drive belt at the same speed as the driven clutch and subsequent drive train components (which are relatively slower than the rotational speed of the drive clutch and engine speed in an overload condition). This difference in rotational speed will cause the drive belt to slip on the sheaves of one (usually the drive clutch) or both of the clutches. Consequently the belt will burn or wear very quickly and usually will need to be replaced in order for the CVT to return to a properly functioning state.
A common solution to this problem is to use a wet-float centrifugal clutch mounted on the output shaft of the vehicle's engine, located inside the engine crankcase. The centrifugal clutch can engage onto a drum that is connected to the input shaft of the drive clutch on the CVT system. When the engine rotates at a certain rpm the centrifugal clutch engages onto the clutch drum and begins to spin the drive clutch. Under overload conditions the centrifugal clutch will not be able to hold the torque necessary to complete a solid connection between the centrifugal clutch shoes and the clutch drum, thus, not connecting the engine output shaft and the drive clutch input shaft. The slipping action of the centrifugal clutch prevents damage to the drive belt.
However, for engines that do not include a wet-float centrifugal clutch, when it is time to replace an existing CVT unit, the incorporation of a centrifugal clutch is very costly and difficult. Also tuning, repair and replacement of the centrifugal clutch in conventional wet-float systems can be expensive and time consuming.